The present invention relates to a method for detecting decrease in tire air-pressure and an apparatus used therefor. More particularly, the present invention relates to a method for detecting decrease in tire air-pressure and an apparatus used therefor capable of properly judging that decrease in internal pressure has occurred in any of tires attached to a vehicle equipped with a limited slip differential device (LSD) and generating. alarm accordingly.
It is conventionally known that a dynamic load radius of a tire becomes small in case an air-pressure of the tire has decreased, and that a rotational velocity or rotational angular velocity becomes faster than that of a tire having a normal air-pressure. For instance, there are proposed methods for detecting decrease in internal pressure based on relative differences of rotational angular velocities of tires (see, for instance, Japanese Unexamined Patent Publication No. 100620/1998, Japanese Unexamined Patent Publication No. 66714/1997, Japanese Unexamined Patent Publication No. 164720/1996 and Japanese Unexamined Patent Publication No. 145654/1996). Among these techniques, apparatus A as recited in Japanese Unexamined Patent Publication No. 100620/1998 is so arranged that in case it is judged that a wheel of decreased pressure is one of following wheels when the vehicle is running on a comer, a turning radius is introduced from a rotational angular velocity of the driving wheels, acceleration in a lateral direction (hereinafter called xe2x80x9clateral Gxe2x80x9d) at the time of running on a corner is calculated from the turning radius, and a judgement value is calculated on the basis of a lateral G including no errors by using a specified threshold.
However, among recent vehicles, there are some which are equipped with a limited slip differential device at a differential gear of a driving shaft for the aim of improving running performance at the time of, for instance, cornering. The mechanism of the limited slip differential device is so arranged that differential is restricted until a differential torque exceeds a set value, and the driving wheels are rotated at identical velocities on the right and left. Therefore, in a vehicle equipped with such a mechanism, a rotational angular velocity at an outer side of turning becomes slower than an essential rotational angular velocity due to influences of the limited slip differential device so that the turning radius is calculated to be larger than it actually is. Consequently, errors occur in judgement values so that decrease in internal pressure cannot be properly judged.
The present invention has been made in view of these facts, and it is a purpose of the present invention to provide a method for detecting decrease in tire air-pressure and an apparatus used therefor capable of properly judging decreases in internal pressure of a tire in a vehicle equipped with a limited slip differential device.
In accordance with the present invention, there is provided an apparatus for detecting decrease in tire air-pressure for detecting decrease in internal pressure of a tire based on rotational information obtainable from tires attached to a four-wheeled vehicle equipped with a limited slip differential device, wherein. the apparatus includes a shift amount calculating means for correcting a turning radius or a reciprocal of the turning radius of driving wheels by using a shift amount which is a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels and following wheels.
In accordance with the present invention, there is further provided a method for detecting decrease in tire air-pressure for detecting decrease in internal pressure of a tire based on rotational information obtainable from tires attached to a four-wheeled vehicle equipped with a limited slip differential device, wherein the method includes the steps of introducing a difference between turning radiuses or a difference between reciprocals of turning radiuses obtainable from the rotational information of driving wheels and following wheels, performing correction by using the difference between turning radiuses or the difference between reciprocals of turning radiuses as a shift amount for the turning radiuses or reciprocals of the turning radiuses of the driving wheels, and thereafter judging decrease in internal pressure of a tire.